Self-contained power units, such as electro-hydraulic actuators, typically include an electric motor that drives a hydraulic pump to move fluid between a reservoir and a hydraulic actuator. The hydraulic actuator generally includes a tubular barrel in which a piston having a piston rod moves linearly, back and forth. The piston seals separate the inside of the barrel into two chambers, a fluid chamber and a piston chamber. The fluid chamber generally is filled with a substantially incompressible hydraulic fluid, typically an oil.
The pressure of hydraulic fluid pumped into or out of the fluid chamber moves the piston within the barrel. In general, when the electric motor is driven in a first rotational direction, the hydraulic pump moves the fluid into the fluid chamber of the hydraulic actuator and out of the piston chamber, thereby extending a piston rod from the actuator housing. When the electric motor is driven in a second rotational direction, opposite the first rotational direction, the hydraulic pump moves the hydraulic fluid into the piston chamber and out of the fluid chamber, thereby retracting the rod.
As the piston rod extends and retracts, the hydraulic pump moves fluid into and out of the reservoir, for example, through one or more inlet/outlet ports located at the bottom of the fluid reservoir. In some systems, the volume of fluid flowing into or out of the reservoir is displaced with a volume of compressible gas within the reservoir. However, if this compressible gas is able to enter the pump, then poor pump performance (e.g., cavitation) and/or the inability of the piston rod to hold a load may occur. Therefore, some systems may provide a bladder within the reservoir that separates the pocket of gas from the pump inlet/outlet to prevent the gas from entering the pump.
In some systems, the bladder is free floating within the reservoir. If the bladder comes into contact with the pump inlet/outlet, then the bladder may obstruct fluid flow to the pump and cause degradation in pump performance, or even destroy the bladder. In other instances, the bladder may seal against the interior surfaces of the reservoir, causing some of the hydraulic fluid to become trapped behind the bladder, which makes the fluid inaccessible to the pump. This may cause the pump to draw a vacuum, which prevents the pump from supplying hydraulic fluid to the actuator and may also cause cavitation.
Still another concern is that the pressure of the compressible gas within the bladder may change over time due to cyclical loading and/or environmental factors. If the pressure of the compressible gas within the bladder drops below a certain level, a partial vacuum may be created within the reservoir that may cause the detrimental effects and poor pump performance discussed above. Such a change in the required reservoir pressure may be particularly problematic because, once the electro-hydraulic actuator has been assembled and installed, it is generally very difficult to replace or maintain.